A thin-film transistor (TFT) substrate of a liquid crystal display device including a TFT as a switching element may include a transparent substrate, a gate electrode, a gate insulating film, a semiconductor layer, a source electrode, a drain electrode, a protective insulating film, and a pixel electrode. The gate electrode may be provided on the transparent substrate. The gate insulating film may be provided on the gate electrode. The semiconductor layer may be provided on the gate insulating film. The source electrode and the drain electrode may be provided on the semiconductor layer. The protective insulating film may be provided on the source electrode and the drain electrode. The pixel electrode may be connected to the source electrode or the drain electrode. A region of the semiconductor layer between the source electrode and the drain electrode may serve as a channel-formation region where a channel is formed.
As a method for fabricating the liquid crystal display device described above, there is known a so-called four-photo-step fabrication method in which the TFT substrate is fabricated in four photolithography steps (hereinafter, also referred to as “photo-steps”) (see Japanese Patent Application Laid-open No. 2002-90779).
As an example of the four-photo-step fabrication method, there are conducted a first photo-step for forming the gate electrode, a second photo-step for forming the semiconductor layer, the source electrode, and the drain electrode, a third photo-step for forming a contact hole through the protective insulating film, and a fourth photo-step for forming the pixel electrode to be connected to the source electrode or the drain electrode through the contact hole.
In the four-photo-step fabrication method, the semiconductor layer, the source electrode, and the drain electrode may be formed in the single photo-step. Therefore, the semiconductor layer may be located below all of the source electrode and the drain electrode. In this case, the semiconductor layer, which extends beyond the range of the gate electrode when viewed in a plan view, may receive light from a backlight unit and may generate alight leakage current. When the light leakage current is generated, there is a risk in that an OFF-leakage current may become high to generate a residual image.
Therefore, there is a need for a display device that can suppress the generation of such light leakage current.